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Coronavirus antigen test: result in a short time
Photo: GEORG HOCHMUTH / AFP
Things have become a bit confusing on the coronavirus test market.
The common PCR tests are now familiar to many people - but now everyone is suddenly talking about rapid antigen tests or gargle tests for schoolchildren, which are supposed to save more stringent measures.
At the same time, there are concerns about the reliability of such procedures.
The overview:
PCR, rapid test, self-test: What test procedures are there?
There are now three test methods for the detection of Sars-CoV-2, each of which is used for different purposes.
PCR:
In the polymerase chain reaction, the RNA of the virus is multiplied in the laboratory and the genetic material - if present - is detected.
The test thus states whether a person was infected with Sars-CoV-2 at the time the sample was taken.
Some of the sample material can be taken by the tested person himself, then this is called a
self test
(see point 2).
Antigen tests:
An antigen test looks for proteins that are typical of the virus.
The analysis material is put on a test strip with Sars-CoV-2-specific antibodies.
If the proteins - also called antigens - are present in sufficient concentration, they bind to the antibodies - the test shows a positive result.
An antigen test, like the PCR, can thus detect an active infection.
Since the result is usually displayed within a short time, the antigen tests are also known as
rapid
tests
.
Antibody tests:
In contrast to the other two test methods, the antibody test can only detect the virus when the infection has subsided.
One week after the illness at the earliest, the immune system produces antibodies that fight the virus in a targeted manner.
These can be detected in the patient's blood serum.
The test therefore says whether someone has been infected with the corona virus in the past.
Since there are test kits that can be used to take a few drops of blood by pricking the finger, these are also known as
self-tests
.
At home or at the doctor's: how are the tests done?
The sample material for the tests can be obtained in different ways.
For the
PCR
, the
Robert Koch Institute (RKI) recommends
that the analysis material be taken from the upper and lower airways in parallel.
Whenever possible, a swab from the nose and throat as well as sputum, i.e. coughed up sputum, or secretion from the bronchi should be examined.
A
nasopharynx swab
or just a throat swab, in which a swab is inserted into the nose or throat to take samples of the mucous membrane
, is usually
carried out
in test centers or at the family doctor
.
Since many find this process uncomfortable, there are a number of other ways of obtaining sample material for the PCR.
However, these are not listed in the RKI recommendations.
One alternative is to test
saliva
.
The Bernhard Nocht Institute for Tropical Medicine in Hamburg, for example, offers a self-test in which the test person puts a so-called suction roll in his mouth for about a minute on an empty stomach in the morning until it is full of saliva.
However, the reliability of this method is controversial (see point 3).
In the current Corona podcast from NDR Info, the virologist Sandra Ciesek describes how one can achieve a relatively meaningful result with a saliva test: The patients had to fill urine cups to a third with saliva.
"You did that in the morning. You didn't eat anything, didn't drink anything, didn't brush your teeth beforehand," she says.
"That's not so little when you get up in the morning and have not had anything to drink or eat, to produce 40 to 50 milliliters of saliva first."
Austria has also been testing a
gargle test
since August in
order to make
taking samples
more pleasant, especially for children: the test person gargles for 60 seconds with a saline solution and then spits it into a sample tube.
The city of Cologne is also currently testing the gargle method.
The result of this validation is to be presented on Friday.
For antibody tests,
blood serum is
required, which is examined for antibodies against Sars-CoV-2.
To do this, blood must be drawn from the test person at the doctor or in the laboratory.
At-home tests that take a few drops of blood from the fingertip are also available online.
The sample must then still be sent to the laboratory for evaluation.
Sars-CoV-2-specific antigens can be detected both in a patient's blood and in the throat, where the virus multiplies.
The Swiss pharmaceutical company Roche, for example, announced at the beginning of September that it would launch a rapid test that would show a result within 15 minutes.
The throat swab is placed on a
test strip
with antibodies.
Since it is very difficult to get deep enough into the throat even with the test stick, the sample can only be taken by medically trained personnel.
Sensitivity and specificity: how reliable are the individual test procedures?
The reliability of the tests depends not only on the procedures themselves, but also on the
quality of the sample taken
.
In general: The sample material should be taken by trained specialists, otherwise the result could be falsified.
The same applies to the transport of the sample, which, according to the RKI, should be delivered to the laboratory as soon as possible after removal and cooled.
The
time at which the sample is taken also
plays a role: it is now known that the viral load is highest two days before and after the onset of symptoms and that a test then provides the most accurate result.
The reliability of individual test procedures is usually given in terms of sensitivity and specificity.
Sensitivity
is the probability that an infected person will be recognized as such.
Specificity
indicates the probability that healthy people actually receive a negative test result.
PCR:
Due to its high reliability, PCR is still considered the gold standard among test methods.
The sensitivity of the PCR tests used in Germany is between
95 and 100 percent
.
However, if the sample material is not obtained via a nasopharynx swab or from the deeper airways, the reliability of the test result also decreases.
Doctor Orth tried the saliva tests at his clinic: "We had high hopes that we could use them to test our employees regularly," he says.
"But when we tried it, we saw that about half of the infected people went undetected."
Antigen tests:
The manufacturer Roche states the sensitivity of its test with
96.52 percent
.
Actually quite high, but according to the statements, the test requires "a sufficient concentration of antigens" in order to identify the sample as positive.
It is now known, however, that especially patients with mild courses often have a lower viral load - and thus also a lower concentration of antigens.
It is questionable whether the test also detects asymptomatic patients or those with only mild symptoms.
Antibody tests:
When detecting antibodies, the most important thing is the right time to take the sample.
This is because the body only produces the long-term IgG antibodies, which are easier to detect, a few weeks after the infection.
So if you take the test too early, you will not get a reliable test result.
But even those who do the test too late could get an inconclusive result.
Because there are also known cases in which the concentration of the antibodies decreases again after a while.
When the test is carried out correctly, some manufacturers indicate a sensitivity of
100 percent
, which means that all people with antibodies are recognized as positive by the test.
The following applies to all test procedures: In addition to the test result, the individual situation of the person tested must always be taken into account.
If someone with a negative test result has had typical corona symptoms and perhaps even additional contact with an infected person, another test should be carried out to confirm the result.
Correct strategy: When is which procedure used?
If tests are used strategically - for example coupled with certain quarantine measures - they can help bring the pandemic under control.
As described above, PCR and antigen tests are designed to detect active infection.
These procedures are used to determine who has been infected with the virus and who can potentially infect others.
Many experts hope that
rapid antigen tests
will soon be usable on a large scale.
It would be conceivable to use them in companies or schools, before train and air travel or before entering care facilities in order to isolate infected people and still not have to restrict healthy people.
Because of their quick results, low cost, and on-site feasibility, they could play a major role in the future.
At the moment, however, the antigen tests are still relatively unreliable: They often only work in patients with a high viral load.
Proponents of such tests are of the opinion that this is still sufficient.
Because even if this only finds patients who are very infectious, so-called superspreader events or the transmission of the virus to risk groups could be better prevented.
"Then this whole diagnostic drama with the long rehearsals and the overloading of the laboratories and all that and the high costs, that is completely solved in one go," said virologist Christian Drosten in early September in the NDR info podcast.
However, he also limited the fact that, due to the low sensitivity of the antigen tests, if the result was positive, the presence of an infection had to be proven again by PCR.
The
saliva or gargle tests mentioned
could also be used in a similar way.
You have the advantage that everyone can take the sample at home.
This reduces personnel costs and the risk of infection.
In addition, the methods are more pleasant than the nasopharyngeal swab.
At the same time, the risk of incorrect sampling increases if this is carried out by laypeople, and laboratory capacities are not spared, since the samples are still evaluated using the PCR method.
Similar to the antigen tests, the informative value of the saliva tests is also limited because they can miss infections.
"But that is acceptable," says the Hamburg virologist Jonas Schmidt-Chanasit from the Bernhard Nocht Institute.
"The key is to find the infectious patient, not all the infected."
And the test is sensitive enough for that.
Due to its high level of reliability, experts such as virologist Sandra Ciesek
still consider
PCR
to be the best method for reliably detecting infections.
The national test strategy describes who should be tested by PCR in Germany: This includes all symptomatic persons as well as contact persons of corona-positives, residents of care facilities, staff in hospitals or care facilities and returnees from risk areas.
In order to save resources, there are already approaches for so-called
pool solutions
in which large groups of people are tested, but several PCR tests are evaluated together.
A PCR is then carried out for each pool; if this is positive, all samples are retested individually.
Systematic and broad testing, for example of residents in nursing homes, which at the same time does not overload the laboratory capacities, would be conceivable.
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